The present invention relates to a mechanical press of the type used for metal stamping and forming, and in particular to a method and apparatus for maintaining a constant shutheight by compensating for thermal growth of the connections.
Conventional mechanical presses comprise a bed which is mounted to a platform or the floor of the shop, a vertically spaced crown portion in which the drive assembly is contained, and one or more uprights rigidly connected to the bed and crown and maintaining the bed and crown in vertically spaced relationship. The crown contains the drive assembly, which typically comprises a crankshaft having one or more eccentrics thereon and connection arms connected to the eccentrics of the crankshaft at their upper ends and to the slide at their lower ends. The slide is mounted within the uprights for vertical reciprocating motion and may be guided in a number of ways, such as by gibs on the uprights themselves or on guideposts rigidly connected to the bed and crown.
At one end of the crankshaft there may be mounted a flywheel and clutch assembly wherein the flywheel is connected by a belt to the output pulley of a motor so that when the motor is energized, the massive flywheel rotates. When the clutch is energized, the rotary motion of the flywheel is transmitted to the crankshaft thereby causing the connection arms to undergo rotary-oscillatory motion that is transmitted to the slide assembly by means of a wrist pin, for example, so that the rotary-oscillatory motion is converted to straight reciprocating motion. These slides reciprocate in the generally vertical direction or in a slightly inclined direction in the case of an open back inclined press thereby causing the die mounted to the slide to engage stock fed into the press on each downward movement of the slide. The other half of the die set is mounted to a bolster which in turn is mounted to the bed of the press.
As the press operates, frictional heat is generated at each place where there is an interface between two moving parts. Examples of sources of frictional heat include the motion between the crankshaft eccentrics and the connection bearings between the crankshaft and the connection arm bearing for the dynamic balancer weight, between the crankshaft and the main bearings, and between the guideposts and their associated bushings. Although much of this heat is dissipated by the oil recirculation system and directly to the ambient, the press itself, particularly the elements of the drive assembly, experience an increase in temperature. This temperature increase is particularly troublesome with regard to the connections between the crankshaft and the slide because the increase in temperature results in a thermal expansion of the connections thereby increasing their length. As the connections increase in length, the shutheight of the press, which is the distance between the slide and bolster at the bottom of the slide stroke, decreases.
If the press shutheight is adjusted to the desired level when the press is cold, then as the press warms up, faulty parts will be produced because of the overextension of the stroke. Conversely, if the press shutheight is adjusted for operating temperatures, then faulty parts will be produced during the thermal warmup period. When performing precision coining and embossing operations, strict maintenance of the press shutheight is imperative. Although the press can be run for a period of time to warm it up to the normal operating temperature, this may require several hours and needlessly expends energy. Interim adjustments in the press shutheight could be made during operation, but this would result in considerable press down time with a concomitant loss of production.
By causing the press uprights to elongate at the same rate as the connections, the thermal growth of the connections could be compensated for and the press shutheight would remain stable. Although the press uprights increase in temperature over time as the press warms up, they do so at a much lower rate than the connections due to their substantially larger mass and exposure to the ambient. Furthermore, the uprights are located at positions remote from the source of the frictional heat, which is generated primarily by the drive assembly located in the crown.
One prior attempt to cause the uprights to elongate in order to compensate for the thermal growth of the connections comprises placing in the uprights thermal heaters of the electrical resistance type. In addition to causing a potential fire hazard or the danger of burns to the operator, the electric heaters were not satisfactory because of the control circuitry necessary to regulate their operation. Because the connections can heat up at different rates depending on the ambient temperature, the effects of the press sound enclosure, and the like, it would be necessary to monitor the temperature of the connections or the shutheight and then regulate the electric heaters accordingly. Due to the existence of a number of points at which malfunctions could occur, systems of this type have not proven to be satisfactory. An additional drawback is that they require an external source of energy to energize the electric resistance heaters.
The problem which has occurred in the past in connection with presses of the general type described above is that of lateral expansion of the crown area at a rate faster than the expansion of the bed. Since the crown contains the moving parts and the oil circulation, it will naturally expand at a higher rate than will the bed, which contains few, if any, moving parts. The effect of this uneven expansion was to disrupt the parallelism of the gib surfaces on which the slide was guided. The solution utilized to overcome this problem was to pump oil from the crown down into the bed so that it would also experience thermal expansion thereby alleviating the gib surface misalignment.